1. Field of the Invention
The present invention relates to a laser irradiation method using a laser beam and a laser irradiation apparatus for performing the laser annealing (apparatus including a laser and an optical system for guiding a laser beam output from the laser to a member to be processed). Further, present invention relates to a method of manufacturing a semiconductor device fabricated by the steps including the laser annealing step. Note that the semiconductor device mentioned through the specification includes an electro-optical device such as a liquid crystal display device or a light emitting device and an electronic device including the electro-optical device as its component.
2. Description of the Related Art
In recent years, a wide range of researches have been made as to the art of applying laser annealing to a semiconductor film formed on an insulating substrate such as a glass substrate to crystallize the semiconductor film or to improve the crystallinity thereof. Incidentally, in the present specification, a crystalline semiconductor film assumes a semiconductor film having a crystalline region including a semiconductor film that entire surface is crystallized.
As compared with synthetic quartz glass substrates, glass substrates have the advantages of being inexpensive and rich in workability and of facilitating fabrication of large-area substrates. The reason why lasers are preferentially used for crystallization is that the melting points of glass substrates are low. Lasers can give high energy to semiconductor films without increasing the temperatures of substrates to a great extent. In addition, lasers are remarkably high in throughput compared to heating means using electric heating furnaces.
Since a crystalline semiconductor film formed by the application of laser annealing has high mobility, the crystalline silicon film is used to form thin film transistors (TFTs). For instance, the thin film transistors are widely used in an active matrix type liquid crystal display device in which TFTs for pixel portion and TFTs for pixel portion and driving circuits are fabricated on one glass substrate.
A laser beam oscillated from an excimer laser or the like is often used as the laser beams. The excimer laser has the advantages of can be high output and irradiate repetitively at a high frequency. Further, laser beams emitted from the excimer laser has the advantages of having a high absorption coefficient with respect to silicon films, which is often used as a semiconductor film. In the laser irradiation, a method in which a laser beam is formed by an optical system so as to have a rectangular shape on an irradiation surface or on an periphery thereof, and then irradiation of laser beam is performed as moving the laser beam (or the irradiation position of the laser beam is made to move relative to the irradiation surface) is preferably used because it has good productivity and is industrially superior. Note that, in this specification, a laser beam having a rectangular shape on an irradiation surface or a periphery thereof is called a rectangular shape beam and a laser beam having a dot shape is called a dot shape beam.
In the other hand, the use of large-area substrate is advanced increasingly. The reason is why a semiconductor device such as a plurality of liquid crystal display device panels is fabricated by using a large-area substrate is that high throughput is obtained and the reduction of the cost can be realized. For instance, a substrate of 600 mm×720 mm, a circular substrate of 12 inches (approximately 300 nm in diameter) or the like are used as the large-areas substrate. In addition, it is thought that a substrate of 1 m×1 m or more is be used in the future.
To form the laser beam, KrF (of wavelength 248 nm) and XeCl (of wavelength 308 nm) are used as exciting gases with respect to the excimer laser used for a laser annealing in general. However, gases such as Kr (krypton) and Xe (xenon) are very expensive and encounter the problem that as the frequency of gas replacement becomes higher, a greater increase in manufacturing cost is incurred.
Attachments such as a laser tube for effecting laser oscillation and a gas purifier for removing unnecessary compounds generated in an oscillation process need to be replaced every three or six years. Many of these attachments are expensive, resulting in a similar problem of an increase in manufacturing cost.
As described above, a laser irradiation apparatus using an excimer laser beam surely has high performance, but needs extremely complicated maintenance and also has the disadvantage that if the laser irradiation apparatus is used as a production-purpose laser irradiation apparatus, its running costs (which mean costs occurring during operation) become too high.
Therefore, it is thought that laser annealing of a semiconductor film is performed by using a solid state laser that the maximum output is highly improved recently. The solid state laser is basically capable of outputting laser beam provided that a solid state crystal, a resonance mirror, and a light source for exciting the solid state crystal are present and therefore there is very little maintenance time and effort compared to excimer lasers. Namely, the running cost is extremely low compared to that of an excimer laser, and therefore it becomes possible to greatly lower the manufacturing costs of semiconductor devices. Further, the availability ratio of a mass production line is increased if the amount of maintenance decreases, and therefore the overall throughput in the manufacturing process increases. This also greatly contributes to a reduction in the manufacturing costs of semiconductor deices. In addition, the surface area occupied by the solid state laser is small compared to that occupied by excimer lasers, and this is advantageous in design of the manufacturing line.
Furthermore, a YAG laser (normally means Nd: YAG laser), a YVO4 laser, a YLF laser, a YAlO3 laser, a glass laser, a ruby laser, an alexandride laser, a Ti:sapphire laser, etc. are used as this solid laser. A YAG laser is explained as an example of the solid laser here. It is known that the YAG laser outputs a laser beam having a wavelength of 1065 nm as the fundamental wave. The absorption coefficient of this laser beam with respect to silicon films is extremely low, and therefore the laser beam as it is cannot be used in the crystallization process of the amorphous silicon film, which is one of the silicon films because of the large energy loss and bad efficiency. However, the laser beam can be modulated into having a shorter wavelength by using a non-linear optical element. Due to the modulated wavelengths, the laser beam is named a second harmonic (wavelength 532 nm), a third harmonic (wavelength 355 nm), a fourth harmonic (wavelength 266 nm), and a fifth harmonic (wavelength 213 nm). The absorption coefficients of these harmonics with respect to amorphous silicon films are very high, and therefore these harmonics are used for crystallization of the amorphous silicon films.